Sulfonic fluoropolymers containing pendant groups terminating with sulfonic groups have been described in the prior art including, for example, U.S. Pat. No. 4,478,695, issued Oct. 23, 1984: U.S. Pat. No. 4,417,969, issued Nov. 29, 1983; and U.S. Pat. No. 4,358,545, issued Nov. 9, 1982. Such sulfonic fluoropolymers have been found to be useful in hot, aqueous media, such as chlor-alkali cells, when the sulfonic fluoropolymer's equivalent weight is above about 800 and its hydration product is less than about 22,000. These sulfonic fluoropolymers were also taught as being useful for a variety of other membrane separator applications and to be useful as acid catalysts.
When sulfonic fluoropolymers are used as membranes, it is desirable for them to be in the form of films. They should have physical integrity commensurate with the physical demands of the device in which they will be used. In electrochemical devices such as electrochemical cells, physical demands on the membrane vary, depending upon the type of cell and the configuration of the cell. For example, in some cells, electrodes (anodes and cathodes) are spaced substantially apart from each other and have the membrane placed between the two electrodes. In such cell configurations, the membrane functions, more or less, as a free-standing film. Such free-standing membrane films are commonly reinforced to increase their strength. Reinforcement materials which are commonly used include a variety of materials such as woven scrims, and randomly-dispersed fibers. However, even when supported, there are still certain minimum levels of physical integrity which the membrane is required to have. Otherwise, it breaks apart and looses its utility.
There are other types of electrochemical cells which are constructed to have electrodes or current collectors in intermittent or continuous contact with a membrane positioned between an anode and a cathode. Such cells are generally referred to as zero gap or solid polymer electrolyte cells. In such cells, the electrodes provide physical support such that membrane films with less physical integrity can be used, whether reinforced or not.
Physical integrity of ionic fluoropolymers are determined, to a large degree, by the amount of water or solvent the fluoropolymers contain. Thus, a sulfonic fluoropolymer that swells excessively because it absorbs substantial amounts of water or solvent, tends to become gel-like and lose much of its physical integrity, relative to an unswollen sulfonic fluoropolymer. The level of swelling (the level of water absorption) is largely determined by the temperature and the environment. For example, a given sulfonic fluoropolymer may be swollen to different levels by water under different conditions. Pure water at a given temperature swells the sulfonic fluoropolymer more than aqueous, salt-containing electrolytes, which, in turn, swell the sulfonic fluoropolymer more than humid gases. However, it is known that increasing the temperature results in increased swelling (water absorption) in each environment listed above. Thus, a single definition of suitable physical characteristics of a sulfonic fluoropolymer to define its usefulness is difficult, because utility depends upon the use to which the sulfonic fluoropolymer will be put.
The prior art defined sulfonic fluoropolymers as being useful primarily in terms of the water absorption of the sulfonic fluoropolymers in an environment of hot aqueous electrolytes, such as the type of environment found in electrolytic cells, for example chlor-alkali cells. From such considerations were derived the limitations of 800-1500 equivalent weights and hydration products of less than 22,000. In those prior art patents, a sulfonic fluoropolymer having a 798 equivalent weight and having a hydration product of 29,400 was not considered useful because of excessive hydration. (See U.S. Pat. No. 4,358,545, Example 1). However, it has been discovered that such equivalent weight and hydration product limitations are not valid for certain sulfonic fluoropolymers that are described herein.
Another matter of concern in defining usefulness of sulfonic fluoropolymers as membranes, is the chemical requirements in a given application. Thus, a film of a sulfonic fluoropolymer used as a separator in a chlor-alkali cell has two critical criteria that it should preferably meet: electrical conductivity and the ability to reject anions. The sulfonic fluoropolymer chosen for use in such conditions is usually based on a trade-off between the electrical conductivity of the sulfonic fluoropolymer, which is effected by both equivalent weight and water absorption, and the sulfonic fluoropolymer's ability to reject hydroxide ions, which is largely determined by the level of hydration, i.e., the degree of hydration per functional group in the sulfonic fluoropolymer. Under these circumstances, where one desires to minimize the passage of hydroxide ions, one usually chooses a sulfonic fluoropolymer having a higher equivalent weight than if one based his decision strictly upon electrical conductivity of the fluoropolymer, alone. Thus, the physical properties of the fluoropolymer, as determined by swelling, is not the deciding factor in choosing the fluoropolymer for this particular use. Therefore, it would be very advantageous to have fluoropolymers which have high ionic conductivity while yet maintaining physical integrity as a viable selective ion transport medium.
Other applications, such as fuel cells and proton pumps, present entirely different chemical and physical requirements for the membrane. The different conditions result in different levels of swelling of the sulfonic fluoropolymer, than encountered in the hot electrolytes of chlor-alkali cells. In fuel cell or proton pump applications, there is little, if any, requirement for rejection of negative ions. The primary requirement is transport of protons at the lowest possible electrical resistance. Thus, the sulfonic fluoropolymer having the lowest equivalent weight within physical constraints resulting from swelling characteristics is desired.
The prior art U.S. Pat. No. 4,358,545) used the measured water swelling of a sulfonic fluoropolymer in its acid form (-H+) to calculate a hydration product as a means of combining two important sulfonic fluoropolymer properties: hydration as measured in moles of water per functional group and equivalent weight. This criteria continues to appear sufficient to characterize sulfonic fluoropolymers having superior performance characteristics. In addition, this prior art found a surprising difference between hydration characteristics of sulfonic fluoropolymers having short pendant chains as opposed to sulfonic fluoropolymers having longer pendant chains (U.S. Pat. No. 3,282,875).